1. Field of the Invention
The present invention generally relates to methods of interpolating and extrapolating seismic recordings. It particularly relates to such methods, where the seismic recordings are obtained using one or more multicomponent towed marine receiver cables or streamers, and especially for use in multiple suppression in such seismic recordings.
2. Description of the Related Art
In the field of seismic exploration, the earth interior is explored by emitting low-frequency, generally from 0 Hz to 200 Hz, acoustic waves generated by seismic sources. Refractions or reflections of the emitted waves by features in subsurface are recorded by seismic receivers. The receiver recordings are digitized for processing. The processing of the digitized seismic data is an evolved technology including various sub-processes such as noise removal and corrections to determine the location and geometry of the features which perturbed the emitted wave to cause reflection or refraction. The result of the processing is an acoustic map of the earth interior, which in turn can be exploited to identify for example hydrocarbon reservoirs or monitor changes in such reservoirs.
Seismic surveys are performed on land, in transition zones and in a marine environment. In the marine environment, surveys include sources and receiver cables (streamers) towed in the body of water and ocean bottom surveys in which at least one of sources or receivers are located at the seafloor. Seismic sources and/or receivers can also be placed into boreholes.
The known seismic sources include impulse sources, such as explosives and airguns, and vibratory sources which emit waves with a more controllable amplitude and frequency spectrum. The existing receivers fall broadly speaking into two categories termed “geophones” and “hydrophones”, respectively. Hydrophones record pressure changes, whereas geophones are responsive to particle velocity or acceleration. Geophones can recorded waves in up to three spatial directions and are accordingly referred to as 1C, 2C or 3C sensors. A 4C seismic sensor would be a combination of a 3C geophone with a hydrophone. Both types of receivers can be deployed as cables with the cable providing a structure for mounting receivers and signal transmission to a base station. Such cables fall into two distinct categories: one being so-called ocean-bottom cables which maintain contact with the sea-floor, while the second category is known as streamers which are towed through the water without touching the sea-floor.
Presently, the seismic industry is in the process of developing multi-component cables or streamers. Multicomponent streamers include a plurality of receivers that enable the detection of pressure and particle velocity or time derivatives thereof. In so-called dual sensor towed streamers, the streamer carries a combination of pressure sensors and velocity sensors. The pressure sensor is typically a hydrophone, and the motion or velocity sensors are geophones or accelerometers. In the U.S. Pat. No. 6,512,980, entitled “Noise reference sensor for use in a dual sensor towed streamer”, issued Jan. 28, 2003, in the name of the inventor Frederick J. Barr (“the '980 Patent”), a streamer is described carrying pairs of pressure sensors and motion sensors combined with a third sensor, a noise reference sensor. The noise reference sensor is described as a variant of the prior art pressure sensor.
In the United Kingdom patent application GB 0402012.9, there is proposed a streamer having a plurality of compact clusters of hydrophones. The streamer is adapted to provide gradient measurements of pressure, which in turn can be readily transformed into particle velocity data.
The main motivation for developing multi-component streamers has been to decompose the recorded data into its up- and down-going components, i.e., to free the data of “ghosts” caused by reflection at the sea surface.
On the other hand, the seismic industry has since long experienced the need to interpolate or extrapolate trace recordings into areas void of receivers. Normally the wavefield and/or its derivatives are only known at a number of discrete locations. However, in practice it is often desirable to extend the knowledge of the wavefield to other points using interpolation, extrapolation or a combination of extrapolation arid interpolation, sometimes known as intrapolation. Such techniques are applied, for example, to determine pressure data along the streamer, away from a streamer, at near-source offsets, or between two adjacent streamers.
One particular undesirable event in marine seismic surveying is the occurrence of what are known as “multiple reflections”, or “multiples.” In a seismic survey, as the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves reflects back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. Seismic waves, however, reflect from interfaces other than just those between subterranean formations, as would be desired.
Seismic waves also reflect from the water bottom and the water surface, and the resulting reflected waves themselves continue to reflect. Waves which reflect multiple times are called “multiples”. Waves which reflect multiple times in the water layer between the water surface above and the water bottom below are called “water-bottom multiples”. Water-bottom multiples have long been recognized as a problem in marine seismic processing and interpretation, so multiple attenuation methods based on the wave equation have been developed to handle water-bottom multiples. However, a larger set of multiples containing water-bottom multiples as a subset can be defined. The larger set includes multiples with upward reflections from interfaces between subterranean formations in addition to upward reflections from the water bottom. The multiples in the larger set have in common their downward reflections at the water surface and thus are called “surface multiples”.
A variety of techniques are known to the art for “suppressing” or “attenuating” multiples in seismic data. Some of these techniques employ a form of interpolation in the manner mentioned above. For instance, some techniques predict what the multiples will look like from the collected data and then eliminate the effects of the multiples in the data. That is, some multiple suppression techniques interpolate “inline” data, or an estimate of data that would have been acquired at other locations on the streamer had receivers been positioned at those other locations.
However, these interpolation techniques fail to interpolate in the crossline direction, leaving a paucity of data in some areas where it is desired. Consider, for instance, a multiple prediction technique known as Surface Related Multiple Elimination (“SRME”). An excellent review of SRME can be found in Dragoset, W. H. and Jericevic, Z., “Some Remarks on Surface Multiple Attenuation”, 63 Geophysics 772-789 (1988), which is hereby incorporated by reference for all purposes as if set forth verbatim herein. Three dimensional (“3D”)-SRME has been one of the more successful but computationally-demanding processing approaches to the suppression of marine surface-related multiples. The crux of the method is the capture and re-use of the recorded seismic wavefield as a simulated secondary source to predict and subsequently remove the multiple train via the 3D acoustic wave-equation. The Achilles heel of the method in conventional surveys has been the limited crossline aperture of the recorded data. This distorts the full 3D simulation of the multiple wavefield and limits the efficacy of the method.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.